The present invention pertains to the field of electronic power control circuits, and more specifically to dynamic power controls used in ballasts employed in lighting systems.
Conventional electronic power control systems typically employ a closed-loop feedback architecture. Certain of these systems, such as digital power control systems, use an analog-to-digital converter (ADC) which digitizes a Signal provided by a power output sensing means. Digitized data values from the ADC are presented to a digital signal processor (DSP), and when deviations are sensed in the ADC output relative to a reference signal, the DSP causes corrective actions to be taken in the drive circuitry to adjust the output to a desired value. To correctly monitor precise changes in a low amplitude signal, a sensing ADC must necessarily have a high bit resolution.
A problem arises, however, when precision control is required for a time varying signal that is several orders of magnitude smaller than the largest signal to be detected. Specifically, for a system to be able to linearly maintain precise sensing of a small signal and also be able to sense large signals requires the use of expensive ADCs or extra controllable gain stages. Conventional methods used to address this problem either limit the dynamic range of the signal or limit the precision of the error detection, both of which degrade the performance of the power system.
In non-power systems applications, such as telephony systems and filters, the aforementioned problem has been addressed using companding in which the amplitude of an analog signal is compressed and the signal is then digitized. The digitized signal is then expanded such that the amplitude of the resulting digital signal is equal to the amplitude of the analog signal, thus making the analog-to-digital conversion transparent to the system. An analog compression technique which uses logarithms is particularly suited to a reduction in the dynamic range of a signal to a more manageable range that can then be sampled with a less expensive, lower bit resolution ADCs. See Y. Tsividia,xe2x80x9cExternal Linear Time-Variant Systems and Their Applications to Companding Signal Processors,xe2x80x9d IEEE Transactions Circ. Syst. II, Vol. 44, pp. 65-85, February 1997; J. Bellamy, Digital Telephony, Wiley Series in Telecommunications, Edition 1990, pp. 108-115. See also U.S. Pat. No. 4,903,020 to Wermuth, et al., and U.S. Pat. No. 5,023,490 to Gittinger. To date companding has not been used in power system applications. Accordingly, it is an objective of the present invention to use companding in power systems.
A system and method for the dynamic power control of a ballast, wherein a compander is used to reduce the dynamic input range requirements of an analog-to-digital converter (ADC) in power systems having a dynamic signal range that is several orders of magnitude smaller than the largest signal being sampled. The compander is comprised of: an analog compressor to create a reduced-amplitude analog signal from an uncompressed analog signal; a sampling ADC to convert each one of the plurality of signal samples of the compressed analog signal to a digital data value; and a data expander to digitally expand the digital data value to an amplitude identical to the amplitude of the uncompressed analog signal.
An algorithm is then used to calculate a correction signal from the expanded digital data values. The correction signal causes a power feedback control circuit (not shown) to adjust the output power level of the load device (e.g. a discharge lamp) to a desired value. The control circuit can be implemented to control either an output current or an output voltage and provides a lower cost and less complex power control system than conventional prior art systems used for such purposes.